Presently available resists usually have one or more drawbacks. For example, most negative acting resists have limited resolution because they swell upon development or, in the case of positive acting, novolac resin based resists, are formulated from materials which are opaque at the shorter wavelengths needed for high resolution work, or are very insensitive as in the case of positive acting, polymethyl methacrylate based resists.
Some problems and recent developments associated with high resolution lithography are described, for example, by M. J. Bowden (ACS Symposium Series 266, American Chemical Society, Washington, D.C. 1984, p. 39-117).
Processes have been developed to circumvent these problems (multi-level resists), but they are complex and require an increase in the number of steps per device layer. An ideal resist would be a single layer which could be exposed with deep UV light (250 to 300 nm) (DUV), X-rays, or electron beam radiation.
In addition to high resolution, which is a measure of how small a structure can be created in the resist, high contrast (defined for a resist as a measure of the resist's sensitivity to changes in exposure dose) is a desirable feature for high resolution resists. This is true because the image formed by the exposure tool is not perfect but has a contract (defined for the tool as the sharpness of its image) limited by the laws of optics and the size of the structure to be reproduced. The resist must correct the fuzziness of the projected image to give a structure of proper size with sharp vertical walls. This is particularly important for structures having widths less than 11/2 .mu.meter, since the resist film is typically 1 to 11/2 .mu.meter thick. If the resist wall is not vertical, any subsequent process step which erodes a portion of the resist surface, such as reactive ion etching, will change the width of the structure which in turn can have a deleterious effect on the circuit being produced. The higher the contrast of a resist, the more the resist can correct for low contrast in the projected image, and the smaller will be the features of an image which can be successfully exposed to give a usable structure.
High sensitivity is also an essential property of a high resolution resist. The economics of semiconductor device manufacture require high throughput. This means that the time taken to expose each wafer must be minimized. As a result, the exposure energy per unit area of resist is constrained. Furthermore, some exposure tools have less total energy available in the bands used to achieve high resolution exposure bands. This further reduces the energy available at the wafer surface.
The use of cationic photoinitiators (such as those disclosed in U.S. Pat. Nos. 3,981,897; 4,450,360; or 4,374,086) to cleave polymer pendant groups so as to change the polymer structure to an extent sufficient to create significantly different solubility characteristics in the irradiated and unirradiated areas is disclosed in U.S. Pat. No. 4,491,628. The polymers described as useful in U.S. Pat. No. 4,491,628 are blocked poly-4-hydroxy styrenes, a blocked poly 4-vinylbenzoate, blocked poly-isopropenylphenyloxyacetates or blocked poly-methacrylates. The blocked groups said to be useful included a wide range of acid labile groups, such as trityl, benzyl or tert-butoxy. This includes structures such as ##STR3## where X and Y are acid labile blocking groups, e.g., ##STR4## for X and --C(CH.sub.3).sub.3 for Y. The structures of the deblocked polymers present after irradiation and baking are ##STR5## respectively.
In short, there is a need for new resists having high sensitivity, high contrast and high resolution. The need arises from the desire of semiconductor manufacturers to produce structures smaller than 11/4 .mu.m in width.
One embodiment of the invention to be disclosed provides a new simplified method of producing positive photoresists having a profile suitable for the technique known as the "metal lift-off method." This method is used to produce narrow lines of metal on a surface by first creating a pattern in a positive photoresist and then depositing metal over the pattern by evaporation. The photoresist film is removed by a solvent, leaving the desired lines of metal on the original substrate. However, if the walls of the pattern are covered with metal, the photoresist cannot be dissolved and consequently methods have been developed to create a profile which prevents the walls from being coated with metal. Viewed from above, the top of the photoresist film should overhang the walls below so that they are shaded from metal deposited from above. Unfortunately, such "T-shaped" or overhang profiles are not normally produced by positive photoresists which are likely to have exposed walls. Obtaining overhanging walls has involved additional steps in the process as will be seen.
One method in the prior art is known as the chlorobenzene soak method (M. Hatzakis, et al., IBM J. Res. Develop., Vol. 24, No. 4, July 1980, pp. 452-60 and Y. Mimura, J. Vac. Sci Technol. 84(1), Jan/Feb 1986, pp. 15-21). In this method a layer of novolac based positive photoresist on a substrate is soaked in chlorobenzene. While chlorobenzene is not a good solvent for novolac resin, it will penetrate the upper portion of the photoresist layer. During development of the photoresist pattern in aqueous alkaline developer, this top portion of the resist layer has a slower dissolution rate than the rest of the layer due to increased hydrophobicity imparted by the chlorobenzene soak. As a result, at the end of development, the patterns have the necessary T-shaped or overhand profile where the thickness of the overhand is the penetration depth of the chlorobenzene. It can be seen that this technique has drawbacks which include the use of a hazardous solvent, increased opportunity for device contamination during the soak, and the addition of extra process steps which increase process complexity.
Another method for producing T-shaped or overhang profiles in a positive photoresist is to use image reversal techniques, such as those found in U.S. Pat. No. 4,104,070 and in L. F. Thompson, C. G. Willson and M. J. Bowden, ACS Symposium Series 219, 117 (1983). These techniques cause a positive acting photoresist to produce the inverse or negative tone of the image mask. That is, those areas of photoresist that would normally be removed by the development step remain instead and those areas that would normally remain are washed away. Since the positive photoresist would produce a positively sloped sidewall during normal processing, it will produce a negatively sloped sidewall if the image is reversed. While this technique produces the requisite line profile, it too adds extra processing steps and, therefore, complexity to the metal lift-off process. (Use of image reversal for lift-off found in: Microcircuit Eng. 84, [Proc. Microcircuit Eng. 84 Conf.] 1984, pp. 203-11.)
A third general method of producing T-shaped or overhang profiles in the imaging layers is by use of multilayer resist systems and isotropic or over etching of the bottom layer. A general discussion is found in ACS 219, pp. 301-305. Specific multilevel systems are disclosed in U.S. Pat. Nos. 3,873,361; 4,004,044 and 4,024,293. In multilevel systems, several layers (usually 2 or 3) of different resists are applied to the substrate. The layers are chosen to have different properties so that the top layer can be patterned without affecting the bottom layer. Then the pattern in the top layer is transferred into the bottom layer(s) by a second step which is usually an etch. Again, the differences in the resist layers allows the bottom layer to be patterned without much change in t e top layer. The conditions during the transfer of the pattern into the bottom layer can be chosen such that the dimension of the opening in the bottom layer is larger than that in the top layer. This can be done by over etching or by use of isotropic etching or by overexposure if the bottom layer can be photoimaged. The top layer(s) of resist will form an overhang since the dimension of the bottom opening is larger. This method incurs all the difficulties attendant to multilevel resist systems such as extra steps, increased defect density and interlayer mixing as well as the difficulties associated with the etching step(s).
As can be seen, there is still a need for a positive photoresist system which will provide a T-shaped or overhang profile adequate for use in the metal lift-off process without the need for complicating steps, hazardous solvents, or difficult-to-control etches.